This invention relates to a device for measuring rotary speed and is of the type which has an optical fiber coil formed of a plurality of winding layers. The optical fiber constituting the coil has two ends, each of which being connected to a beam splitter. Two light beams, after being introduced at the coil ends and passing in opposite directions through the optical fiber coil, are superimposed and the phase shift caused by the Sagnac-effect is evaluated.
An rpm measuring device of the above-outlined type is known and is disclosed, for example, in U.K. Patent Application No. 2,146,428. The optical fiber coil described therein has two ends which are arranged either in the same winding layer or in adjoining winding layers of the coil. It is further a requirement in this prior art structure that both fiber ends be situated closely to one another to establish a symmetry with respect to the fiber coil ends. It has been found, however, that thermal interferences as well as radiation effects have an influence on partial coil zones (external circumference) as well as on the entire coil. Stated differently, such interferences may cause, in the prior art construction of the coil, non-uniform effects on the light rays because in the prior art coil construction a symmetry is ensured only with respect to the coil ends. Such interferences significantly affect the accuracy of the measuring results.